This invention relates to improvements in matrix decoding apparatus intended for the periphonic reproduction of sound, using encoded information signals. Many types of decoding apparatus for this purpose exist in the prior art, particularly in relation to four-channel decoding of surround sound encoded by phase and amplitude matrixing onto two channels for transmission or recording using stereophonic media.
In the multichannel decoding apparatus according to the prior art, there are both fixed matrix decoders and variable matrix decoders. Fixed matrix decoders are those in which a plurality of input signals containing encoded information relating to the directions of sound sources are summed in appropriate proportions and phases to yield a plurality of output signals suitable, after amplification, for driving a corresponding plurality of surrounding loudspeakers in a room, the process being describable in terms of a matrix transformation in which the matrix coefficients are fixed and time-invariant. The optimum performance of such decoders occurs when the decoding matrix is the pseudo-inverse of the encoding matrix, as shown by J. V. White in his paper "Synthesis of 4-2-4 Matrix Recording Systems," J. Audio Eng. Soc., Vol. 24, No. 5 , pp. 250-257, May 1976. Such a decoder is said to be matched and no further improvement in its performance is possible unless the coefficients can be varied dynamically.
Variable matrix decoders also matrix a plurality of encoded input signals to produce a plurality of output signals suitable for driving a multichannel loudspeaker system, but the decoding matrix coefficients do not remain fixed. Instead, they are varied by means of a directionality sensing and control system, which continually monitors the correlations in phase and amplitude ratios between the input signals and adjusts the decoding coefficients to provide the maximum possible enhancement of directional cues for the most prominent sound sources at any instant of time. Typical of such decoders are those of Scheiber, U.S. Pat. No. 3,632,886; Bauer, U.S. Pat. No. 3,708,631; ITo and Takahashi, U.S. Pat. No. 3,836,715; Kameoka, et al., U.S. Pat. No. 3,864,516; Tsurushima, U.S. Pat. No. 3,883,692; Gravereaux and Budelman, U.S. Pat. No. 3,943,287; Willcocks, U.S. Pat. No. 3,944,735; Olson, U.S. Pat. No. 4,018,992; and Gerzon, U.S. Pat. No. 4,081,606. While the detailed circuitry and methods used to implement the variation of decoding matrix coefficients in these and numerous other matrix decoders differ, all of the known decoder systems utilize means for determining from the encoded signals present at their input terminals the predominant components of the sound field, deriving therefrom a number of control signals, which are in turn used to vary gain parameters of the decoder and thereby modify the decoding coefficients to optimize the directional cues in the reproduction of those sounds.
Because the control signals and the corresponding decoder matrix coefficients in such systems vary in time, careful attention must be paid to psychoacoustic performance, including phenomena which are known to those skilled in the art such as breathing or pumping effects, mislocalization or apparent wandering of sound sources, modulation of noise associated with the signals, fluctuation of the total sound level, harmonic and intermodulation distortion caused by too rapid variations in the control signals and other effects.
Undesirable effects can also occur because of imperfections in the performance of decoders and also of the encoders and media used to transmit, store or reproduce the information.
This invention represents a new approach to the optimization of the dynamic characteristics of the control signals present in decoding apparatus of the types described in the known patents so as to maximize the desired directionality enhancement effects while minimizing the undesirable effects mentioned above. It may be incorporated into any such decoder system with beneficial results.
In a well-designed control signal generator in such a decoder, the control signals and their sum can be assumed to behave in a manner that will result in correct separation, localization and placement of individual predominant sound sources. However, under dynamic conditions, if the control signals are permitted to vary fast enough to follow all the variations of predominant directionality, the resulting presentation will accentuate intermodulation distortion effects and rapid localization changes and any localization instability due to noise or rumble. The intermodulation distortion is reduced if the time constants are made longer, but the effects of slow image localization shifting, breathing or noise modulation and pumping are more evident under these conditions.
Some of the prior-art decoder systems have attempted to address this problem. Willcocks, in U.S. Pat. No. 3,944,735, "Directional Enhancement System for Quadraphonic Decoders," describes an attack and decay time constant processor section wherein each control signal is stored on a capacitor which is discharged at a variable rate depending upon the relative strength of other control signals present. The attack time constants are always short, and are unaffected by the other control signals. While such a time-constant processing circuit does have some benefits, a side-effect is that the sum of the control coefficient signals can exceed the optimum level causing more severe level variations and deterioration of the sharpness of localization under some circumstances.
Kameoka, et al., in U.S. Pat. No. 3,864,516, "Four-Channel Stereophonic Sound Reproduction System," includes a circuit which normalizes the sum of the control coefficients to a predetermined value at all times, thereby minimizing this effect. However, without a time-constant processor, this circuit may actually accentuate some others of the undesirable effects listed above.